For the fabrication of organic, light-emitting devices (also referred to as electroluminescent devices and, in abbreviation, as EL devices), it is important that the thin film preparation method is scaleable to substrates of various sizes and particularly to large area substrates for economic reasons. Among various fabrication methods, physical vapor deposition has been widely used because it is relatively simple and is capable of producing highly efficient devices.
Turning to FIG. 1, which shows a schematic view of a prior art method of making organic layers for organic light emitting devices. A substrate 10 which is to receive an organic layer, is positioned adjacent to an aperture mask 12. The aperture mask 12 provides an aperture shown as a dimension A over a portion of the substrate 10. An organic material 3 which is to provide a layer on or over the substrate, is disposed in a source crucible 4 which is heated by suitable arrangements such as shown heating coils 5. When heat is applied to the crucible 4, the organic material vaporizes in a reduced pressure environment, such as in a reduced pressure chamber 1 having a pump port 2. The vapor emanates from the crucible 4 as schematically indicated by the dotted vapor arrows 3v and condenses as a deposited layer 3d, also depicted in dotted outline, on portions of the chamber 1, the underside of the aperture mask 12, on the substrate 10 through the aperture A in mask 12, on the underside of a shutter blade 7 and associated shutter shaft 8, as well as on the surfaces of deposition monitors 9a and 9b.
In order to fabricate highly efficient organic light mitting devices it is preferred to sequentially deposit on the substrate 10 a plurality of relatively thin (approximately 100-500 .ANG.) organic layers of different organic materials, as will be detailed below. Prior to forming each of these layers on the substrate, the shutter shaft 8 is rotated such that the shutter blade 7 is positioned to shield the substrate 10 until such time as is needed to initiate and stabilize the vaporizing of an organic material 3 from the source crucible 4, measured by the deposition monitors 9a, 9b. During this start-up and stabilizing period, the above-mentioned layers 3d of organic material are formed within portions of the chamber 1, but not on the substrate. Upon opening the shutter blade 7 to the position shown in FIG. 1, an organic layer 3d is formed on the substrate, and the layers elsewhere in the chamber keep growing in thickness. Such growth of layer thickness within the portions of the chamber and its associated parts can lead to cracking, flaking or dusting of these layers during cycles of pressure reduction (pump-down) and pressure increase (venting) of the chamber and during rotation of the shutter between open and closed positions.
It will be appreciated that the generation of these undesirable particulates reduces the yield of devices having consistent quality. Frequent cleaning of the chamber and associated parts can overcome the yield concern, but at the cost of reduced throughput of fabricated devices.
Another disadvantage of this prior art method of depositing organic layers in organic light emitting devices is relatively poor utilization of the organic material 3 in the source crucible 4, i.e., as little as approximately 10-20% of organic material 3 may be utilized to form a layer 3d on the substrate 10, with 80-90% of the material forming undesirable layers elsewhere in the chamber. Since the fabrication of highly efficient organic light emitting devices calls for purified, and therefore relatively expensive, organic materials, poor material utilization is clearly undesirable.
The aforementioned significant disadvantages are magnified when contemplating the fabrication of organic EL devices over relatively large area substrates, for example over substrates as large as 30 cm by 30 cm.